1. Field of the Invention
The invention relates to any process for oligomerizing olefins that can produce a fuel, for example the production of gasoline and/or kerosene from light olefinic feeds containing 2 to 8 carbon atoms, and in particular from light olefinic feeds containing a high proportion of propylene and/or butenes using an oligomerization catalyst based on silica-alumina with a reduced proportion of macropores. Compared with silica-alumina type catalysts already cited in the literature, the activity of the catalyst of the present invention is greater for the processes cited above.
2. Description of Related Art
Processes for oligomerizing light olefins for the production of olefins with a higher molecular weight are widely used in refining and in petrochemistry, with the aim of upgrading light olefins to bases for gasoline, kerosene or gas oil type fuels, or for solvents. Such oligomerization reactions are carried out in the presence of a catalyst, usually a solid catalyst. The olefins combine into dimers, trimers, tetramers, etc, the degree of oligomerization depending on the type of catalyst used and its temperature and pressure conditions of operation. The advantage of the oligomerization process over other processes which are well known in the field of refining and petrochemistry resulting in the same range of products resides in the fact that the compounds obtained contain no sulfur and contain very few aromatic compounds. The solid oligomerization catalysts often cited in the literature are catalysts of the solid phosphoric acid type (for example U.S. Pat. No. 2,913,506 and U.S. Pat. No. 3,661,801), silica-aluminas (for example U.S. Pat. No. 4,197,185, U.S. Pat. No. 4,544,791 and EP-A-0 463 673), zeolites (for example U.S. Pat. No. 4,642,404 and U.S. Pat. No. 5,284,989) and, to a lesser extent, heteropolyanions (for example Indian patent IN 170 903).
Solid phosphoric acid type catalysts have good activity as regards oligomerization, but they are difficult to manipulate, in particular when discharging, as they tend to increase in mass in the presence of olefins. Further, they cannot be regenerated. Heteropolyanion type catalysts produce a limited degree of polymerization as they do not tolerate high temperatures well. Zeolites produce oligomers with a more limited degree of branching than the preceding catalysts because of high form selectivity in the micropores. This is favorable to gas oil production, which has to have the right cetane index, but is not favorable to the production of gasoline which has to have a high octane number. Finally, silica-alumina type catalysts cited in the literature have fairly variable porosities which produce different characteristics. As an example, EP-A-0 463 673 claims, for the oligomerizing propylene, the use of an amorphous silica-alumina with a silica/alumina molar ratio between 30/1 and 500/1, a specific surface area between 500 and 1000 m2/g, a total pore volume between 0.3 and 0.6 ml/g, a mean pore diameter of at most about 1 nm, and with no pores with a diameter of more than 3 nm. U.S. Pat. No. 4,544,791 claims, for the oligomerizing C4 olefins, the use of an amorphous silica-alumina with a silica content in the range 60% to 95% by weight, a specific surface area between 50 and 500 m2/g, and a total pore volume between 0.4 and 0.9 ml/g, but said silica-alumina does not exhibit an alumina phase in X ray diffraction.